For some time, dental prostheses were produced from porcelain veneering material bonded onto a metal framework (substructure, or core). This porcelain-fused-to-metal (“ceramo metal”) construction required a fairly bulbous metal core, with a fairly uniform porcelain thickness to avoid de-lamination under sheer conditions. More recently, dentists have been offered high strength ceramic materials as substitutes for the conventionally metal substructures in ceramo-metal crowns and/or bridges. These porcelain veneer fused to non-metal core crowns/bridges offer reduced labor costs, equivalent or superior precision to ceramo-metal alternatives, improved aesthetics, excellent bonds to the underlying high-strength ceramic sub-structure, reduction in thermal stresses, and reduced thermal conductivity.
Particular emphasis is placed on recent improvements in strength of the various new non-metal substructure materials. Traditional ceramic cores have compressive strengths in the region of 150-200 MPa. Alumina based cores claim strengths in the region of 400-600 MPa, and Zirconia cores have strengths ranging from 900-1200 MPa. Notwithstanding the impressive strengths of these substructure materials, these figures are deceptive due to the more limited strength of the veneering materials used for the crowns and/or bridges. Furthermore, the more aesthetic and much weaker veneering materials are not adequately supported by the higher strength ceramic core, often resulting in fracturing of the veneering porcelain while the ceramic core remains intact.
In restorations supported by implants instead of teeth, the fracture potential may be increased even more. This increased fracture potential is caused in part by an increase in the bulks of veneering porcelain. First, implants tend to be placed more lingually than the teeth they are replacing, which often results in more unsupported buccal porcelain. Second, the diameter of an implant platform is often smaller than the tooth it is replacing, giving rise to still bigger bulks of unsupported porcelain. The situation is further complicated by implants being more rigid than teeth, while at the same time resulting in reduced proprioceptive capabilities. All these factors tend to concentrate and accentuate the forces placed on the veneering porcelain.
Fracture of veneers is not a new problem. Traditionally, in ceramo-metal technology, the weakness of the veneering ceramic material has been compensated for by bulking up and designing the substructure in certain quite well defined ways. Nevertheless, bulking up the substructure often results in a less desirable aesthetic appearance, as the substructure becomes more visible through the veneer. One example of an attempt to provide a concealed support includes constructing a metal collar at the base of the substructure with a shoulder brought part way up the lingual surface of the substructure. Also, in the approximal regions the substructure frame is frequently built out under the contact points with the veneer. The concept behind these design elements is to reduce the bulk of the veneering porcelain and to convert the loading stresses on the veneer ceramic from being in shear to being under compression.
Many operators are applying the same concepts traditionally used for ceramo-metal technology to the design of high strength ceramic substructures. Notwithstanding, these design elements are not much help in strengthening the most visible portions of the teeth, the buccal surfaces. This can be a particular problem in the mandible because in a normally related occlusion, the buccal cusps of the mandibular teeth serve as occlusal supports. Using a high strength ceramic substructure, which tends to be opaque and of high luminosity, half way up the buccal surface to reinforce the veneer porcelain of the cusp is just as unacceptable as using a metal frame in the same manner. Furthermore, a marginal collar at the approximal regions of the substructure provides little or no reinforcement because it is so far away from the region where the stress is being applied (i.e. the tip of the buccal cusps).
Therefore, it would be beneficial to provide a strengthening mechanism for a crown and/or bridge close to the region where the stress is being applied to the veneer that does not compromise aesthetics and which is relatively simple to design and construct.